[Qemu-devel] [RFC PATCH 0/7] BlockBackends, nodes and guest devices

[Kevin Wolf]

I am relatively confident to say that everything that should use a
BlockBackend, does so by now. Almost all users create their own anonymous
BlockBackend internally and use that. The user configures the BB only
indirectly using the configuration methods of the user that the BB belongs to.

There is one exception, which are guest devices. There the user is expected to
manually set up a BlockBackend and pass it to the device. This requires that
users understand the difference between node and BlockBackends and manage both
kinds of objects. This is a rather nasty interface.

My goal is that we allow a user (management tool) to ignore that BlockBackends
exist as separate entities in qemu. Ideally we could fully make them an
implementation detail, but I'm not sure to which degree we can do that for
compatibility reasons. But what I'm pretty sure we can do is provide interfaces
that address everything using either node names or (qdev) device names, so that
you don't have to manage BlockBackends if you don't want to.

This involves several steps, and for most of them this series contains an
example patch that shows what this could look like:

1. Accept node-name in -device drive=... and create an internal anonymous BB
   for devices configured this way. This is the way to create devices that
   completely avoid legacy interfaces using the BB name.

2. Update all QMP commands touching block devices. There are two kinds of them,
   concerning either the guest device (which the BlockBackend is logically part
   of, even though it's not implemented this way) or the actual backend
   (BlockDriverState/node level)

   * Device level commands: Accept a guest device ID instead of BB name to
     identify the BlockBackend the command works on. As device IDs and BB names
     don't share a single namespace, we'll need a new QMP argument for this.

   * Node level commands: We need to complete the conversion that makes
     commands accept node names instead of BlockBackend names. In some places
     we intentionally allow only BlockBackends because we don't know if the
     command works in other places than the root. This is okay, but we can
     accept node names anyway. We just need to check that the node is a root
     node as expected.

3. Remove all BlockBackend options from blockdev-add. This has already happened
   partially (e.g. WCE flag), but at least id, rerror, werror are still there.
   This is a very incompatible change, but we declared blockdev-add
   experimental, so I think it's acceptable.

4. Add BlockBackend options as qdev properties to all block devices.

5. Add a way on the command line to create block nodes that have a node-name
   and don't have a BlockBackend. blockdev-add already supports this (and after
   implementing 3. it will be the only mode supported by blockdev-add), but we
   can't do this on the command line yet. You always get a BB with -drive.

   This might finally become the -blockdev we were talking about at the very
   beginning of the block layer generalisation work.

So this is my plan. It's pretty radical, but I think we really must do
something about our interfaces. Having nodes, BlockBackends and guest devices
to manage is just too much and doesn't really make sense. Making BlockBackends
visible in the external API essentially only as aliases for either node names
or guest devices (and that only for compatibility, not when using blockdev-add/
-blockdev) feels to me like the right thing to do.

But of course I'm aware that there probably isn't a clear right or wrong, and
that I might be missing important details, so this needs to be discussed in
advance before I go and implement the full thing instead of just small example
patches.

So please let me know what you guys think about this plan.

Kevin Wolf (7):
  block/qdev: Allow node name for drive properties
  block: Add blk_by_dev()
  qdev-monitor: Factor out find_device_state()
  qdev-monitor: Add blk_by_qdev_id()
  block: Accept device model name for blockdev-open/close-tray
  block: Accept node-name for block-stream
  block/qdev: Allow configuring WCE with qdev properties

 block/block-backend.c            | 19 +++++++++
 blockdev.c                       | 92 ++++++++++++++++++++++++++++------------
 hw/block/block.c                 | 16 +++++++
 hw/block/nvme.c                  |  1 +
 hw/block/virtio-blk.c            |  1 +
 hw/core/qdev-properties-system.c | 18 +++++++-
 hw/ide/qdev.c                    |  1 +
 hw/scsi/scsi-disk.c              |  1 +
 hw/usb/dev-storage.c             |  1 +
 include/hw/block/block.h         |  5 ++-
 include/sysemu/block-backend.h   |  2 +
 qapi/block-core.json             | 16 ++++---
 qdev-monitor.c                   | 34 +++++++++++++--
 qmp-commands.hx                  | 14 +++---
 14 files changed, 178 insertions(+), 43 deletions(-)

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1.8.3.1